ABSTACT Basal-like breast cancer is estrogen receptor (ER), progesterone receptor (PR) and Her2 negative; it is not responsive to hormone therapy and drugs that target the HER2 protein. Therefore, it is urgent to develop novel therapeutic strategies for basal-like breast cancer. Basal-like breast cancer is rich in tumor-initiating cells. Tumor initiating cells are drivers of cancer metastasis and relapses; thus targeting tumor-initiating cells is critical for basal-like breast cancer treatment. Our recent study shows that 3WJ RNA nanoparticles incorporated with EGFR aptamer successfully bind and enter into the basal-like breast cancer cell line MDA- MB 231 in culture and in the orthotopic mouse mammary tumor model. RNA aptamers targeting the tumor initiating cell marker CD44 have been identified and characterized, which offers a potential strategy for delivering the 3-way junction (3WJ) RNA nanoparticles into tumor initiating cells. We and others recently demonstrate that miR-29b is a potent tumor suppressor for basal-like breast cancer. Expression of exogenous miR-29b in basal-like breast cancer cell lines significantly inhibits cancer metastasis in orthotopic mammary tumor models and repressed malignant phenotypes in 3D culture. MiR-29 also demonstrate potent inhibitory activity on tumor-initiating cells. Therefore, we predict that therapeutic delivery of miR-29b will inhibit basal-like breast cancer progression and metastasis. The overall objective of this proposal is to develop a novel approach to therapeutically deliver tumor suppressor miRNA into basal-like breast cancer. In combining with RNA aptamers to target cell surface markers of basal-like breast cancer and 2?F modification to enhance miRNA stability, the RNA nanoparticle provides a powerful tool to therapeutically deliver miR-29 to basal-like breast cancer. The central hypothesis of this proposal is that tumor suppressor miR-29b can be therapeutically delivered to basal-like breast cancer tissue using 3WJ RNA nanoparticles, and subsequently represses cancer progression and metastasis. To test our central hypothesis and achieve the objective of this proposal, we have designed experiments with the following specific aims. Aim 1. Construct RNA nanoparticles harboring therapeutic miR-29b that will target basal-like breast cancer cells. Aim 2. Examine the biologic activity of multifunctional therapeutic RNA nanoparticles in the orthotopic mouse mammary tumor model. Development of RNA nanoparticles that deliver miR-29b to basal-like breast cancer tissue and tumor-initiating cells will overcome the current hurdle in utilizing miRNA for cancer therapy. Examining the potential of exogenous miR-29b in inhibiting progression and metastasis of basal-like breast cancer may identify a potential therapeutic strategy for this deadly disease.